The variable pulley transmission, or continuously variable transmission (CVT) as it is commonly called, has been under development for over two decades, but its use as a power transmission is mainly for automotive applications. Its control is rather complicated and is very sensitive to operating conditions. For example, in a control arrangement for a variable pulley transmission disclosed in U.S. Pat. No. 4,458,318, a variable line pressure is controlled and applied to the sheave of the secondary or driven pulley of the transmission, and also serves as the hydraulic power supply to a ratio control valve in a speed control loop of the transmission.
It has found that due to the variable supply pressure in the continuously variable transmission of U.S. Pat. No. 4,458,318, both the pressure control valve (a pressure relief type valve) and the ratio control valve gains change with the supply pressure, and the control is therefore very sensitive to system operating conditions. There is a need for an improved continuously variable transmission with a control arrangement and method which overcome these drawbacks and disadvantages of this known continuously variable transmission. More particularly, there is a need for an improved continuously variable transmission with a control arrangement and method which are simpler and more robust than those disclosed in U.S. Pat. No. 4,458,318. It is an object of the present invention to provide a continuously variable transmission with a control arrangement and method which meet this need.
The commonly owned U.S. Pat. No. 6,099,424 for CONTINUOUSLY VARIABLE TRANSMISSION WITH CONTROL ARRANGEMENT AND METHOD FOR CONTROL ARRANGEMENT AND METHOD FOR REDUCING TRANSMISSION BELT SLIPPAGE, filed concurrently with this application and hereby incorporated by reference. However in the application of a CVT as a constant speed drive for an electrical power generator, if a large load is suddenly applied, the belt speed may start to slip. If this happens too often, damage of the belt and pulley may occur. One way to entirely avoid belt slippage would be to apply very large forces to the sheaves at all times so that even the largest possible load can be accomodated. This would definitely shorten the life of the belt. The present invention aims to provide an improved continuously variable transmission having a control arrangement and method whereby just enough forces need be applied to the sheaves to prevent the belt from slipping under all possible load conditions.
These and other objects are attained by the continuously variable transmission of the present invention for transferring drive from an engine to a device to be driven wherein the transmission comprises a primary pulley for receiving drive from an engine, a belt, a secondary pulley which is coupled over the belt to the primary pulley for transferring drive to a device to be driven, each of the primary and secondary pulleys having an axially movable sheave and a hydraulically operated actuator therefor to effect ratio change of the transmission and to maintain belt tension, and wherein a single source of constant hydraulic pressure is operatively connected for driving the actuators. In the disclosed form of the invention, the source of constant hydraulic pressure comprises a hydraulic pump and a pressure relief valve in communication with the output of the hydraulic pump to maintain a constant predetermined hydraulic output pressure of the pump. The single source of constant hydraulic pressure is operatively connected to the actuator of the primary pulley by way of a ratio control valve and to the actuator of the secondary pulley by way of a pressure control valve.
According to further features of the invention, the continuously variable transmission includes a hydraulic pressure control loop for controlling the hydraulic pressure applied to the actuator of the secondary pulley as a function of the sensed load of the device to be driven on the transmission and the pitch radius of the secondary pulley. An output speed control loop is also provided for controlling the output speed of the transmission driving the device to be driven. In the disclosed embodiment the output speed control loop may be set to control the output speed at a constant value for driving an aircraft electric generator. According to another feature of the invention, the hydraulic pressure control loop measures the speed of the generator and the load current of the generator being driven for calculation of the load torque of the generator on the transmission.
The CVT of the invention also includes a load feed-forward control loop for feeding back a load signal, proportional to the load torque of the generator on the transmission to each of the hydraulic pressure control loop and the output speed control loop to speed up the responses thereof with a sudden increase in the load torque. In addition a torque lagging apparatus is provided in the transmission between the secondary pulley and the generator to insert a lag in the transmission to allow the control loops time to respond to a sudden increase in load torque for preventing belt slippage.
A method of the invention for reducing belt slippage in the case of a sudden increase in load torque of the generator on the transmission inserting a lag in the transmission between the secondary pulley and the generator to allow the control loops time to respond to the sudden increase in load torque, and speeding up the response of the control loops in response to the sudden increase in load torque.